Patella and its function in knee ; Q angle


The primary function of the patella is to increase the moment arm of the quadriceps muscle in its function to extend the knee. It also redirects the forces exerted by the quadriceps.


Knee Function

Patellar Alignment

The alignment of the patella in the frontal plane is influenced by the line of pull of the quadriceps muscle group and by its attachment to the tubial tubercle via the patellar tendon. The result of these two forces is a bowstring effect on the patella, causing it to track laterally. One method of describing the bowstring effect is to measure the Qangle. The Q-angle is the angle formed by two intersecting lines: one from the anterior superior iliac spine to the midpatella, the other from the tibial tubercle through the midpatella .A normal Q-angle, which tends to be greater in women than men, is 10_ to 15_

Forces Maintaining Alignment

In addition to the bony restraints of the trochlear groove (femoral sulcus), the patella is stabilized by passive and dynamic (muscular) restraints. The superficial portion of the extensor retinaculum, to which the vastus medialis and vastus lateralis muscles have an attachment, provides dynamic stability in the transverse plane. The medial and lateral patellofemoral ligaments, which attach to the adductor tubercle medially and iliotibial band laterally provide passive restraints to the patella in the transverse plane. Longitudinally, the medial and lateral patellotibial ligaments and patellar tendon fixate the patella inferiorly against the active pull of the quadriceps muscle superiorly.


Patellar Malalignment and Tracking Problems

Malalignment and tracking problems of the patella may be caused by several factors that may or may not be interrelated.


Increased Q-angle

 With an increased Q-angle there may be increased pressure of the lateral facet against the lateral femoral condyle when the knee flexes during weight bearing. Structurally, an increased Q-angle occurs with a wide pelvis, femoral anteversion, coxa vara, genu valgum, and laterally displaced tibial tuberosity. Lower extremity motions in the transverse plane that may increase the Q-angle are external tibial rotation, internal femoral rotation, and a pronated subtalar joint. Functional knee valgus that occurs during dynamic activities also increases the Q-angle

Muscle and fascial tightness

 A tight iliotibial (IT) band and lateral retinaculum prevent medial gliding of the patella. Tight ankle plantarflexors result in pronation of the foot when the ankle dorsiflexes, causing medial torsion of the tibia and functional lateral displacement of the tibial tuberosity in relationship to the patella. Tight rectus femoris and hamstring muscles may affect the mechanics of the knee, leading to compensations.

Lax medial capsular retinaculum or an insufficient VMO muscle

The vastus medialis obliquus (VMO) muscle may be weak from disuse or inhibited because of joint swelling or pain, leading to poor medial stability.225 Poor timing of its contraction, which alters the ratio of firing between the VMO and vastus lateralis (VL) muscle, may lead to an imbalance of forces. Weakness or poor timing of VMO contractions increase the lateral drifting of the patella.

Hip muscle weakness

 Weakness of the hip abductors and external rotators may result in adduction of the femur and valgus at the knee under loaded weight bearing.

Patellar Compression

Patellar contact

 The posterior surface of the patella has several facets. It is not completely congruent as it articulates with the trochlear groove on the femur. When the knee is in complete extension (0_), the patella is superior to the trochlear groove. By 15_ of flexion the inferior border of the patella begins to articulate with the superior aspect of the groove. As the knee flexes, the patella slides distally in the groove, and more surface area comes in contact. Beyond 60_ there is controversy as to whether the contact area continues to increase, level off, or decrease.80,81 In addition, as the knee flexes past 90_, the quadriceps tendon comes in contact with the trochlear groove as the patella slides inferiorly.

Compression forces

 In full extension, because there is minimal to no contact of the patella with the trochlear groove, there is no compression of the articular surfaces. Furthermore, because the femur and tibia are almost parallel, the line of pull of the quadriceps muscle and patellar tendon causes a very small resultant compressive load. The resultant force of the quadriceps and patellar tendon forces rises as the knee flexes, but there is also greater surface area of the patella in contact with the groove to dissipate this force. The joint reaction force on the articular surface rises rapidly between 30_ and 60_. There is controversy as to the extent of joint reaction forces in greater degrees of flexion. During squatting, the joint reaction force continues to rise until 90_ and then levels off or decreases because the quadriceps tendon begins making contact with the trochlear groove and therefore dissipates some of the force. In an open-chain exercise with a free weight on the distal leg, the greatest joint reaction force occurs at around 30_ of flexion.80 This is because of the changing moment arm of the resistive force more than the line of pull of the quadriceps and patellar tendons. In an open-chain with variable resistance, the peak stress is at 60_ and peak compression at 75_.56 An increased Q-angle causes increased lateral facet pressure as the knee flexes

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